1. Field of the Invention
The present invention relates to a lens meter which measures optical characteristics of a lens to be examined.
2. Description of Related Art
Conventionally, there is known a lens meter having a measurement optical system which projects a measurement light bundle onto a lens to be examined, detects the measurement light bundle transmitted through the lens by a photodetector, and obtains optical characteristics (sphere power S, cylinder power C, and an astigmatic axial angle A) of the lens based on results of the detection. Such a conventional lens meter is configured such that the optical characteristics of the lens are measured based on deviations of a group of four measurement targets (three measurement targets in principle) which are arranged around a measurement optical axis, the deviations detected by the photodetector (see Japanese Patent Application Unexamined Publication No. Sho60-17335 and U.S. Pat. No. 3,880,525 corresponding to Japanese Patent Application Unexamined Publication No. Sho50-145249). In addition, a lens meter is proposed which uses a plurality of measurement targets arranged in a nosepiece in order to facilitate measurement of distribution of optical characteristics of a lens and a distance portion and a near portion of a progressive power lens (see U.S. Pat. No. 6,972,837 corresponding to Japanese Patent Application Unexamined No. 2003-75296). As for any lens meters, measurement of a single vision lens is performed basically by using measurement targets arranged on a circumference of a circle having a diameter of 2 to 3 mm around the measurement optical axis, because the further the measurement targets are arranged from the measurement optical axis, the greater influence of aberration.
However, in measurement based on the measurement targets in the vicinity of the measurement optical axis, measurement of optical characteristics become unstable depending on power of a lens and conditions of a lens surface, which may cause unfavorable reliability of measurement accuracy. In other words, when refractive power of the lens is weak, deviations of the measurement targets in the vicinity of the optical axis are small, and thereby measurement values tend to be unstable. Specifically, when the cylinder power is weak, the astigmatic axial angle influenced thereby greatly varies, causing unstable results of the measurement and low measurement accuracy. In addition, in the measurement using the measurement targets in the vicinity of the measurement optical axis, when there is a scratch or soil in a measurement region, measurement values become unstable, causing unfavorable reliability of measurement accuracy.